What are the three practical uses of isotopes?

one of the most important is to check out the macanism of reaction. if u use an isotope, for example of carbon or oxigen, u can see where it goes, so to understand the way that the reaction follows!then u can use in NMR Deuterio instead of hydrogen, to be sure of the structure of molecules.mmm if something more comes in my mind, i'm gonna writing!byee